Expansion valves as known from U.S. Pat. No. Re. 23,706; U.S. Pat. Nos. 4,819,443 and 4,979,372 control the flow rate of a refrigerant supplied to an evaporator by means of a valve mechanism which is driven by the displaceable diaphragm wall forming one wall of a temperature-sensing chamber. The valve mechanism opens or closes a supply passage for the refrigerant. The temperature-sensing chamber contains at least a saturated vapor gas responding by pressure changes to temperature changes in the refrigerant returning from the evaporator. The temperature-sensing chamber is either provided in the return passage or at an exterior side of the expansion valve housing. Within the temperature-sensing chamber, the diaphragm surface has a lower temperature than the other confining walls so that the saturated vapor gas at least partially condenses and liquefies on the diaphragm wall surface. Depending on the position of the expansion valve, the liquefied part of the saturated vapor gas can contact other and warmer wall portions of the temperature-sensing chamber, and starts to evaporate and gasify again, resulting in a rapid rise of the pressure in the temperature-sensing chamber. Since the pressure of the saturated vapor gas attributable to the diaphragm surface temperature is lower than the pressure of the saturated vapor gas, the gas again condenses on said diaphragm wall surface. As a result, the pressure in the temperature-sensing chamber periodically fluctuates which leads to an actuation of the valve mechanism. Accordingly, the refrigerant flow rate towards the evaporator fluctuates uninterruptedly. This leads to an unstable refrigeration cycle in the refrigerating system. Furthermore, if the position of the expansion valve is changed in an uncontrolled manner, for example, in a moving vehicle the refrigeration cycle may be varied constantly even if cooling demand remains unchanged.
Moreover, the valve opening curve of an expansion valve depends entirely upon the properties of the sealed charge in the temperature-sensing chamber. It is difficult to set a desired ideal valve-opening curve in cases where the sealed charge is only a saturated valve gas identical or similar in nature to the refrigerant being controlled.
Furthermore, when minute changes of the temperature of the refrigerant returning from the evaporator are transferred to the sealed charge in the temperature-sensing chamber too rapidly, minute pulsations result in the refrigerant flow. Such minute changes in the superheat of the refrigerant directly cause the valve mechanism to open and to close and lead to an unstable expansion valve operation. Such temporary changes in the refrigerant temperature at the return side of the evaporator unavoidably occur even during normal operation of the refrigerating system. However, these minute and transient temperature changes should not be considerably affect the operation of the expansion valve.